The Organic Light Emitting Diode (OLED) display device has characteristics including self light-emitting, low driving voltage, high emitting efficiency, short response time, high resolution and contrast, about 180 vision angle, wide temperature range, great flexibility, image imprinting for a larger panel size, etc. Thus, it is considered a prospecting display device.
Pixels of the OLED display device are arranged in a matrix, and a pixel driving circuit can drive the OLEDs to emit lights. The driving method of the OLED display device includes the analog driving method and the digital driving method. When using analog driving method, TFTs of different pixels may have different characteristic parameters, so different pixels may have different driving currents with the same driving voltage, which causes the Mura (i.e. the bad brightness uniformity of the OLED display device). By using the digital driving method, the Mura can be reduced.
Referring to FIG. 1, a circuit diagram of a 3T1C pixel driving circuit of an OLED display device according to prior arts is shown. As shown in FIG. 1, the 3T1C pixel driving circuit includes a first TFT T1, a second TFT T2, a third TFT T3, a storage capacitor Cst and an OLED D. The second TFT T2 is a driving TFT. The gate of the second TFT T2 is connected to a first node A, and the source of the second TFT T2 is connected to a second node B. The first TFT T1 is configured to charge the node A (i.e. the gate of the second TFT T2), and the third TFT T3 is configured to discharge the node A (i.e. the gate of the second TFT T2).
When the 3T1C pixel driving circuit drives the pixels of the OLED display device, the first TFT T1 charges the node A and the third TFT T3 discharges the node A, so two Gamma voltages are outputted from the node A (i.e. the gate of the second TFT T2). These two Gamma voltages are the maximum Gamma voltage GM1 that leads the maximum brightness of the OLED and the minimum Gamma voltage GM9 that leads the minimum brightness of the OLED. The current I flowing through the OLED can be calculated according to the following equation.I=k(VGS−Vth)2=k(VA−VB−Vth)2 
In this equation, k is an eigen conductive factor of the second TFT T2, VGS is the gate-source voltage of the second TFT T2, Vth is a threshold voltage of the second TFT T2, VA is the voltage at the node A (i.e. the voltage at the gate of the second TFT T2), and VB is the voltage at the node B (i.e. the voltage at the source of the second TFT T2).
The variation ΔVth of the threshold voltage Vth is smaller than VA−VB because the element degradation or bad uniformity. Thus, compared with the analog driving method, the digital driving method can improve the brightness uniformity of the OLED display device.
The first TFT T1 charges the node A and the third TFT T3 discharges the node A, so two Gamma voltages are outputted from the node A. The brightness of the OLED display device is adjusted in a way similar to the Pulse-Width Modulation (PWM) to have gray scales. Referring to FIG. 2, a schematic diagram of light-emitting segments and blank segments of each of eight sub frames of each frame according to prior arts is shown. In FIG. 2, 8bits OLED display device is taken as an example. Each frame is sequentially divided into eight sub frames. The gray scale brightness can be determined by controlling the charging time and the discharging time of the sub frames. Thus, different sub frames can output different gray scale, and digital driving signals are accordingly generated. In FIG. 2, the time ratios of driving the pixels in the sub frames (i.e. the first sub frame SF1 to the eighth sub frame SF8) are 1/128:1/64:1/32:1/16:1/8:1/4:1/2:1. The eighth sub frame SF8 is a full-emission sub frame, and in the full-emission sub frame, pixels of the OLED display device keep emitting lights.
Generally speaking, each frame includes a driving period TD and a blanking time TB. The driving period TD is defined as the time consumed when scan lines in an effective display area are sequentially driven, and the blanking period TB is defined as the time interval between one driving period TD and next driving period TD. Referring to FIG. 3, a schematic diagram of time segments of each of eight sub frames of each frame according to prior arts is shown. The driving period TD and a blanking time TB are together divided into eight sub frames. In other words, the driving period TD is divided into eight segments and each segment belongs to one sub frame, and the blanking time TB is also divided into eight segments and each segment belongs to one sub frame. Therefore, each frame is TD/8+TB/8. In the eight sub frames, the pixels do not emit lights near the blanking time TB, and thus the pixels do not emit lights in the blanking time TB. Referring to FIG. 4, another schematic diagram of light-emitting segments and blank segments of each of eight sub frames of each frame according to prior arts is shown. In FIG. 4, the 8 bit gray scale is taken as an example. The time for the 255 gray-scale pixel having the maximum brightness to emit lights is about 25% of one frame. In other words, the maximum brightness generated by using the digital driving method is only 25% of the maximum brightness generated by using the analog driving method. Thus, the brightness of the OLED display device using the digital driving method is pretty low.